Access to a manhole assembly is typically by means of a manhole cover which is supported within a cover frame positioned a spaced distance above the top opening in the manhole assembly and maintained at grade. One or more adjustment rings, typically referred to as grade rings, are arranged upon the top of the manhole assembly to provide support for the cover frame and to maintain the cover frame at grade level. The region occupied by the grade rings is highly susceptible to water infiltration and it is important to provide means and apparatus to prevent water from infiltrating into the adjustment region between the cover frame and the top of the manhole assembly to thereby prevent water from infiltrating into the interior of the manhole assembly.
Techniques for preventing water from infiltrating into the adjustment region are described in U.S. Pat. No. 4,737,220, issued Apr. 12, 1988 and U.S. Pat. No. 4,621,941 issued Nov. 1, 1986 and assigned to the assignee of the present invention. The aforementioned patents disclose a water lock chimney-type structure which, in addition to preventing water from infiltrating into the adjustment region, protects the interior adjustment region from corrosion within the system when sealed against the cast iron support frame thereby preventing hydrogen sulfide gas from contacting the concrete adjustment area.
Although the water lock arrangement disclosed in U.S. Pat. No. 4,737,220 has been very successful when all installation conditions are followed and met, the water lock structure has the disadvantage of wrinkling when the masonry work surrounding the rubber sleeve has a smaller inner diameter than the outer diameter of the sleeve, said wrinkling significantly reducing the access clearance for the manhole.
The above problem has been alleviated by placing an expandable stainless steel clamping band or ring at the top of the sleeve to prevent the rubber sleeve from protruding away from the masonry wall by urging the sleeve outwardly and clamping the sleeve against the masonry wall. The retention ring thus locks the sleeve to the surface directly behind the ring whereby the ring may be dislodged if any movement occurs to the surface behind the ring due to thermal expansion or contraction created by extreme temperature changes of the soil surface such as freeze or thaw cycles and further due to any vibration originating either upon or below the surface. The movement of the clamp may cause the clamp to dislodge and fall into the channel area of the manhole base which could result in a blockage of the flow of waste material in the system sewage, causing a back flow within the collector system. The clamping band may also contribute to a reduction in the access clearance opening.
The chimney lock assembly of U.S. Pat. No. 4,737,220 also necessitates being cast into the manhole assembly and may not be conveniently employed for installation in situ and has a limited expansion range.